This invention relates generally to an optical fiber connector, and, more particularly, to an optical fiber connector which causes light energy transmitted by the core of a transmitting fiber to be expanded over a larger area in order to reduce losses due to misalignment with the core of a receiving fiber.
It is well known in the art that information bearing light can be caused to propagate along a transparent fiber structure which has a higher refractive index than its surroundings. To be effective, excessive attenuation of the transmitted light must be avoided. Within the fiber, these losses may result from one or more causes, such as scattering and absorption; however, many of the problems in providing practical low loss glassy materials and production techniques for low-loss fibers have been largely overcome. An article entitled "Fiber Optics" by Narinder S. Kapany published in Scientific American, Volume 203, pages 72-81, November 1960, provides useful background with respect to the theoretical and practical aspects of fiber optic transmission, and a further detailed discussion at this time is not deemed necessary.
Notwithstanding the above, if optical fibers are to be used as practical signal transmission media, practical, low-loss connectors for coupling the optical fibers must be provided. The primary consideration is one of transfer efficiency. That is, in order to reduce light losses between connected optical fibers, the fiber ends must be precisely aligned both axially and angularly so that there is no separation at the point of abutment and no lateral separation (axial misalignment) as these will result in a loss of light energy at the connector, thereby reducing the connector's transfer efficiency. This problem is further discussed in the Bell System Technical Journal, Volume 50, No. 10, December 1971 in an article by D. L. Bisbee entitled "Measurement of Loss due to Offset and End Separation of Optical Fibers".
When one considers that an optical fiber may have a core diameter in the order of a few microns and an overall diameter of, for example, 100 microns, the difficulty in providing alignment when connecting two optical fibers can be appreciated, especially when one considers that connecting must often be accomplished out in the field by installers, repairmen and the like, without the aid of complex laboratory and precision aligning equipment. This prospect of precision aligning and connecting optical fibers which may be as small and flimsy as human hairs would frustrate even the most dextrous of technicians.
Contributing to the overall problem of misalignment when connecting optical fibers is the fact that typically, the optical fiber core and its outside cladding are not concentric. Clearly then, even if the fibers could be perfectly aligned, the cores themselves might well remain misaligned, resulting in loss of transmitted energy.
One known device for aligning optical fibers is shown and described in U.S. Pat. No. 3,768,146 issued Oct. 30, 1973 and comprises a base plate having V-shaped grooves therein for holding the fibers. A metallic sleeve is crimped over adjacent fiber ends by a compression plate to secure and align the fibers within the groove. A second known device, shown and described in U.S. Pat. No. 3,734,594 issued May 22, 1973, comprises a deformable, angular core disposed between a pair of metallic pressure plates. The two fibers to be spliced or connected are inserted into opposite ends of the core, and a longitudinal force is applied to the plates. This causes the core to deform radially, simultaneously aligning and mechanically securing the fibers.
Both of the above described known devices do not provide sufficient accuracy for joining and aligning small core optical fibers. An axial displacement of five microns, for example, causes a loss of 1 DB for a 25 micron core fiber. Available displacement tolerances of about .+-.50 microns in conventional devices is therefore highly inadequate. A satisfactory device would require a tolerance of about .+-.2 microns, and any technique for aligning fiber optic cores using a mechanical device presents serious problems.
One approach for providing an optic fiber connector which eliminates high losses due to misalignment includes reducing the need for precision connecting of the fibers. This may be accomplished by expanding the light energy from the fiber core throughout a larger area so that misalignment of the larger area interface in the connector yields a tolerable loss of energy. This approach has been considered in British Pat. No. 1,017,354 published January 19, 1966, wherein the described connector includes a transparent body for coupling the ends of two fibers, the transmitting fiber having a larger core diameter than that of the receiving fiber. The transparent body has a semiellipsoidal shape and consists of a material having the proper light-transmitting properties and an inner reflecting surface which is optically polished and metalized to provide a mirror-like surface. Light emanating from the cores of the transmitting fibers and impinging on the reflecting surface are reflected onto the cores of the receiving fibers. However, to ensure that most of the transmitted energy is received by the receiving fiber, the connecting device has a relatively complex shape, that is, that portion nearest the transmitting fiber of a larger cross-section is cylindrical, that portion nearest the receiving fiber is conical and the intermediate portion ellipsoidal. While this arrangement may be suitable for its intended purpose, i.e., coupling light transmitted from one element to another wherein each element has a different cross-section, it does not provide a simple connecting technique.
A second known arrangement for coupling single optic fibers is described in U.S. Pat. No. 3,995,935 issued Dec. 7, 1976 entitled "Optical Coupler". This patent teaches the use of an optical connector comprising an optical chamber having a reflective wall. The chamber is filled with a light transmitting filler so that light emitted into the chamber by a light-emitting device will reflect off the chamber wall and impinge upon the outer surface of a receiving fiber. Two chamber embodiments are shown. The first is a rotated conical section, and the second a rotated truncated parabola. However, in both cases, the slope of the reflective wall is dependent upon the ratio of the index of refraction of the receiving fiber core to the index of refraction of the chamber filler. This indicates that different connectors having different wall slopes would be required for fibers having cores with different indices of refraction. Since it is well known that the numerous applications for optical fibers in modern technology require fibers having many different indices of refraction, this technique requires the availability of many different connectors having various different shapes.